The present invention relates to a semiconductor structure and a method of fabricating the same and, more particularly, to an island region for a semiconductor device and a method of fabricating the same.
It is pointed out that, when the size of a semiconductor heterostructure is several tens nm or less, which is equal to or smaller than the de Broglie wavelength of carriers (electrons and holes) in a semiconductor crystal, a quantum containment effect is appeared, which is not observed in a bulk crystal.
This is because the carriers are localized in such a small semiconductor heterostructures.
Bulk crystal illustrated in FIG. 27A has the function of the density of states of carriers which is continuous with the carrier energy as shown in FIG. 27B. This situation is shown in FIG. 27A. In a semiconductor with a quantum well film structure shown in FIG. 27C, the density of states of carriers has a stepwise Junction against energy as in FIG. 27D. In a semiconductor with a quantum well film wire structure in FIG. 27E, the density of states of carriers should have a spike form function against energy in FIG. 27F. In a semiconductor with a quantum box structure shown in FIG. 27G, the density of states of carriers should have completely a discrete function as in FIG. 27H.
Because of the recent advances of crystal growth technologies, a thin high-quality semiconductor film or multi-layer films formed by stacking this thin film can be easily fabricated as two-dimensional quantum well films. In these quantum films, an emission spectrum whose full-width half maximum is narrower than hat of a bulk crystal is obtained, and unique physical properties, such as high optical gain and the generation of excitons at room temperature, are observed, due to their localized density of states.
Using these quantum effects, the characteristics of semiconductor lasers or various optical devices such as optical modulators and optical switches have been significantly improved.
By taking into account the success of the introduction of these quantum well film structures, the fabrication and physical property of further localized structures such as quantum wires or quantum boxes are extensively studied at present.
Various methods for the fabrication of a quantum wire structure or the quantum box structure have been proposed. The most conventional method is to form fine patterns on a two-dimensional quantum well film by using electron beam lithography, and these patterns are transferred into a quantum film by etching. The other method is focused ion beam lithography, which can directly form wires or boxes.
In both methods, thin semiconductor films are formed by controlling the compositions and thickness precisely by using epitaxial growth technologies. Thereafter, the size in the lateral direction is controlled by, lithography and/or etching of quantum wires or quantum boxes. Therefore, the minimum size and size uniformity size of the structure in the lateral direction is severely limited by the accuracy of the process technologies used.
Recently, several attempts in which a carrier confined region is formed by using growth technology such as MOVPE (Metal Organic Vapor Phase Epitaxy) or MBE (Molecular Beam Epitaxy) have been reported. These methods make use of the chemical properties of crystal growth. That is, when a thin film is grown on a substrate crystal which is subjected to certain processing, stable facet surfaces appear in a crystal with a three-dimensional structure depending on the growth conditions and the growth methods. A multi-dimensionally confined structure can obtained by the successive Growth of several kinds of epitaxial layers under the different conditions.
The characteristic feature of this method is that the damage and contamination to the carrier confined region caused by the lithographic process can be eliminated.
In order to fabricate quantum wires and quantum boxes using epitaxial growth technology, the selective area growth which semiconductor surface is covered with patterned passivation films has been frequently used, and a wire is grown in the opening of passivation films.
Other examples of conventionally known methods are one in which facets are formed on a crystal face having the shape of a V-shaped groove and a wire structure is formed on the bottom of this V-shaped groove, and one in which a wire structure is formed in the longitudinal direction on the basis of steps formed on a tilted substrate.
As a quantum well film box structure which aims to realize a O-dimensional electron-hole system, a tetrahedral quantum well film box structure is examined, which is fabricated by, e.g., forming an AlGaAs epitaxial film on a (111)B GaAs substrate by performing MOVPE selective-area growth using SiO.sub.2 as a mask.
Unfortunately, in these wire and box structures, it is necessary to form a one-dimensional or two-dimensional structure of several tens nm size not only in the direction of film thickness but in the plane of a thin quantum well film. Therefore, the quality of a crystal degrades more easily than a thin quantum well film. This makes it difficult to realize a quantum wire or a quantum box with optical characteristics comparable to or better than to those of a quantum well film.
In the case of selective growth method, a short diffusion length of Al causes a large selective mask dependence on the thickness and composition of an AlGaAs epitaxial film. This makes the formation of ultra-fine box structure difficult. Also, there are additional problems that the spatial density of the quantum well film boxes is limited by the fabrication processes of a selective mask and this selective mask must be removed in the fabrication of a device (semiconductor device).
As a method of obtaining a multiple quantum wire structure, a (110) cleaved surface of a periodic structure of GaAs/AlGaAs grown on a flat GaAs is used for the growth substrate by using semiconductor epitaxial growth of GaAs/AlGaAs structure quantum wire surface as tile cleavage plane. In this method, the selective growth of GaAs occurs due to a spontaneous oxide film firmed on underlying AlGaAs with a heavily doped Al composition. Consequently, it is possible to realize a multiple GaAs/AlGaAs quantum wire structure of which dimension is defined by the underlying periodic structure of GaAs/AlGaAs.
In this method, however, the formation of electrodes or the like must be performed in a narrow region because the cleavage plane of a crystal is used. Therefore, it is difficult to fabricate useful optical devices using this method.
On the other hand, as a method of forming a quantum well film box structure on a plain semiconductor substrate by using only vapor phase crystal growth, the formation of island-like InP on a GaAs substrate is reported. Since the lattice constant of InP is largely different from that of GaAs, InP is not formed as a film but randomly aggregates into islands on a GaAs substrate.
Unfortunately, a large variation in island size is inevitable in this method. In addition, it is difficult to obtain high density islands because the increase in the amount of InP causes the crystal defect in islands. Therefore, the limited density of island-like InP formable makes the fabrication of optical device on a practical level difficult.